Retroreflector with controlled divergence made by the method of localized substrate stress

ABSTRACT

A cube corner array with minute modifications of the dihedral angles of some of the cube corners is produced by introducing a stress or change via the opposite side of the substrate. The array having aberrated elements can be replicated for use in the manufacture of a tool, and the tool can be used in the manufacture of retroreflective products of broader divergence.

This application is a continuation of U.S. application Ser. No.10/317,812, filed Dec. 12, 2002, U.S. Pat. No. 6,871,966.

BACKGROUND OF THE INVENTION

This invention relates to a method of making a retroreflective articlehaving controlled divergence, and articles made by the method.

It is well known that retroreflective articles can be made from an arrayof microcube corner elements. Such an array of microcube corner elementscan be made by ruling a master of “male” cube corners into a planarsurface of a plate. This is taught generally by Stamm U.S. Pat. No.3,712,706. It also is taught in detail in Pricone U.S. Pat. No.4,478,769, assigned to the common assignee and incorporated herein byreference in its entirety.

U.S. Pat. No. 4,478,769 describes a well-known method of makingtriangular cube corner elements, in which the planar surface of a masterplate is ruled with a diamond cutting tool that cuts a series of preciseparallel vee-grooves. To rule equilateral triangular cube corners, threesets of parallel grooves in directions intersecting one another atangles of 60° are made; each groove also will have an included angle ofsubstantially 70.53° disposed symmetrically, and will be ruled to agroove depth determined by the height of the cube corners desired. Thismethod automatically results in an array of pairs of oppositely orientedequilateral triangular microcubes on the face of the master. To rulenon-equilateral triangle cube corners the grooves within the parallelsets will contain angles other than 70.53°, and intersect at anglesother than 60°, as disclosed, for example in Rowland U.S. Pat. No.3,684,348. Methods for ruling non-triangle cube corners generally do notuse three sets of parallel symmetrically disposed vee-grooves, but thefaces of the cube corners are nevertheless formed from the walls ofgrooves, as disclosed, for example in Nelson U.S. Pat. No. 4,938,563.Methods for creating cube corner arrays other than by ruling a singleplate have been disclosed in U.S. Pat. No. 6,015,214. The presentinvention applies to all microcube corner arrays, regardless of how theyoriginated.

A master of “male” or “female” cube corner elements can be used to makea sequence of copies, of alternating gender, such as by electroforming.At any stage, copies can be assembled together, and the assemblies usedto make further copies. After a series of assembly and copying stages, asingle “mother” can be formed. The “mother” can be used to makeproduction tools, such as by electroforming, which tools can be used toform microcube retroreflective elements on an expanse of plasticsheeting material such as by embossing, casting, compression molding orother methods known in the art.

Microcube corner retroreflective sheeting such as made by the methoddescribed above is used in highway safety applications such as highwaysigns and pavement markers. In such applications, the microcube cornerelements reflect light from a vehicle's headlights back to the eyes ofthe driver of the vehicle. This is an inexact retroreflection in whichthe divergence angle, α, ranges between approximately 0° and more than3°. The value of α operative in any given situation depends on thegeometry of the vehicle and the driver and the distance from the vehicleto the retroreflective material. For example, the divergence angle α fora large truck's right headlight and its driver at a distance of about 40meters from a road sign will be approximately 3°, while the divergenceangle α for an automobile's left headlight and its driver at a distanceof about 600 meters from a road sign will be approximately 0.05°.

Also associated with the divergence angle, α, is a rotation angle, ε,which is a measure of the direction of the divergence. The value of εwill be different for left and right headlights of a vehicle, and willalso depend on the vehicle and driver geometry and the position of theroad sign. For sheeting that will be mounted in random orientation onroad signs, retroreflectance is required at every value of ε. The anglesα and ε are defined in ASTM E808, Standard Practice for DescribingRetroreflection, which document refers to divergence angle α as“observation angle”.

Ideally, microcube corner retroreflective sheeting used in road signswill produce a pattern of retroreflected light having sufficientintensity over a range of divergence angle values and rotation anglevalues. For example, even a non-urban retroreflective highway signshould retroreflect light through a divergence angle α of about 1°,which corresponds to the value of α from a large truck's right headlightback to its driver at a distance of about 120 meters from the road sign.

Improvements in the precision with which microcube corner elements canbe ruled in a master plate and duplicated, particularly by embossing,have led to concerns that such microcube corner retroreflective sheetingmay be adequately retroreflective over only a very narrow range ofdivergence angle, such as about 0.0–0.5 degrees. It would be preferredto provide an array of cube corners producing the entire desired rangeof divergence and within very short distances on the array so that ahuman observer of the article will see it as retroreflectively uniform.

Light that is retroreflected by micro-sized cube corner elements willexperience a certain amount of diffraction because of the very smallsize of the microcubes. Such diffraction will result in retroreflectionover broader ranges of both divergence angle and rotation angle. Theparticular ranges of α and ε will depend on the particular diffractionpattern of a given microcube, which will depend in turn upon the cubesize, cube shape, the index of refraction of the cube material, and uponwhether or not the cube faces have been metallized. Diffraction,however, is not a desirable method to enhance retroreflection throughbroader divergence and rotation angle, because the very small microcubesthat achieve greater diffraction also cause a substantial quantity oflight to be retroreflected with a divergence angle α of greater thanabout 3°, where the light is not useful to the vehicle driver.Diffraction also can result in idiosyncratic diffraction patterns thatare unlikely to distribute the retroreflected light in a manner thatwill be useful to a vehicle's driver.

It is known in the art to produce divergent rectroreflectance by meansof cube corner elements having intentional aberrations in respect oftheir dihedral angles deviating slightly from 90°. The classic paper“Study of Light Deviation Errors in Triple Mirrors and TetrahedralPrisms,” J. Optical Soc. Amer., vol. 48, no. 7, pp. 496–499, July, 1958by P. R. Yoder, Jr., describes the well-known spot patterns resultingfrom such aberrations.

U.S. Pat. No. 3,833,285 to Heenan, assigned to the common assignee andincorporated herein by reference in its entirety, teaches that havingone dihedral angle of a macro-sized cube corner element greater than theother two results in extended observation angularity in microcubes, andspecifically that the retroreflected light diverges in an elongatedpattern.

When ruling an array of cube corners, dihedral angle errors may beproduced either by causing the groove side angles to be slightlydifferent from the design angles, as taught by Stamm U.S. Pat. No.3,712,706, or by causing the angles of groove root crossings to bedifferent from the design angles, or by combination of these methods.

U.S. Pat. No. 4,775,219 to Appeldorn discloses retroreflective articleshaving tailored divergence profiles, wherein the cube corner elementsare formed by three intersecting sets of parallel vee-grooves, andwherein at least one of the sets includes, in a repeating pattern, atleast two groove side angles that differ from one another. Nelson U.S.Pat. No. 4,938,563 extended the method of U.S. Pat. No. 4,775,219 tonon-repeating patterns of groove side angle differences.

U.S. Pat. No. 6,015,214 to Heenan et al., assigned to the commonassignee, teaches methods of forming microcubes by ruling vee-groovesinto the edges of a plurality of flat plates, and discloses that thetilt angle of a cutting tool with respect to the plate edges being ruledcan be adjusted continuously as each groove is cut as a function of thedistance traveled by the cutting tool across the plate edges.

Pending U.S. patent application Ser. No. 10/167,135, filed Jun. 11,2002, claiming the benefit of Ser. No. 60/297,394, filed Jun. 11, 2001,subsequently published as U.S. Patent Application Publication No. US2003/0075815 A1, discloses retroreflective articles and a method ofmaking retroreflective articles having controlled broader divergenceproduced by ruling three intersecting sets of parallel V-shaped groovesin which ruling non-uniform deviations of the cube dihedral angles fromexactly 90° are intentionally introduced by causing the cutting tool andthe surface of the substrate to oscillate with respect to one another ina controlled manner during the ruling of at least one of thevee-grooves.

It is thus one object of the invention to provide an article comprisingan array of retroreflective microcube corner elements having controlledbroader divergence.

It is another object of the invention to provide a method for makingsuch an article.

SUMMARY OF THE INVENTION

In accordance with the method of the invention, a substrate is providedhaving opposed first and second surfaces, said first surface having anarray of cube corner elements. The substrate is worked in a controlledmanner at one or more localized regions on the second surface to createa localized change in the stress of the substrate material. The degreeof working and the arrangement of locations can be substantially regularor they can have controlled irregularity. The substrate is sufficientlythin that the change in stress induced by the working on the secondsurface causes a change in one or more dihedral angles of one or more ofthe cube corner elements on the first surface opposite the region of theworking. The changes in the dihedral angles of the affected cube cornerelements will be on the order of a fraction of a degree. This change issufficient to create an aberration in the cube corner element that willaffect the divergence of light retroreflected by the cube corner. Thearray with one or more aberrated cube corner elements can be copied,assembled, and recopied, as often as desired and used to form a toolsuitable for use in the manufacture of microprismatic sheeting, such asby casting, embossing, compression molding, or other methods. The cubecorner sheeting made from such a tool will have a broader range ofdivergence than cube corner sheeting made from arrays having either noaberrated cube corner elements, or cube corner elements aberratedsubstantially identically by fixed angle deviations. In a preferredembodiment, the total retroreflectance of the array is substantiallypreserved.

DESCRIPTION OF THE FIGURES

The foregoing and other novel features and advantages of the inventionwill be better understood upon a reading of the following detaileddescription taken in conjunction with the accompanying drawings wherein:

FIG. 1A is a top plan view of a substrate having a standard pattern ofretroreflective triangular cube corner elements formed thereon, such asis known in the art.

FIG. 1B is a front view of the substrate of FIG. 1A when the cube cornerelements are males.

FIG. 1C is a front view of the substrate of FIG. 1A when the cube cornerelements are females.

FIG. 2 is a photomicrograph of a cross-section of a metal substratehaving a plurality of cube corner elements on one surface thereof, andsubjected to laser energy at three discrete locations on the oppositesurface.

FIG. 3A is a schematic top plan view of an experimental array oftriangular cube corner elements in which a stress has been introduced atthe center cube corner element.

FIG. 3B is a schematic top plan view of an experimental array oftriangular cube corner elements in which a stress has been introduced atan area between two cube corner elements.

FIG. 3C is a schematic top plan view of an experimental array oftriangular cube corner elements in which a stress has been introduced atan area between six cube corner elements.

FIG. 4A is a graph showing calculated R_(A) values of hypothetical priorart unaberrated cube corner sheeting over a divergence angle range of2°, and for three different angles of rotation.

FIG. 4B is a graph showing R_(A) values of hypothetical prior art cubecorner sheeting made from a master aberrated during ruling in accordancewith the prior art, the R_(A) values being calculated over a divergenceangle range of 2°, and for three different angles of rotation.

FIG. 4C is a graph showing calculated R_(A) values of a hypotheticalsample of a cube corner sheeting made in accordance with the instantinvention in which astress has been introduced once for every 12 cubecorner elements, the R_(A) values being calculated over a divergenceangle range of 2°, and for three different angles of rotation.

FIG. 4D is a graph comparing the average of the three curves of FIG. 4A,the average of the three curves of FIG. 4B, and the average of the threecurves of FIG. 4C.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The term “cube corner elements” as used herein includes those elementsconsisting of three mutually intersecting faces, the dihedral angles ofwhich are generally on the order of 90°, but not necessarily exactly90°.

The term “substrate” as used herein means a thickness of a materialhaving an array of either male or female cube corner elements formed ona first surface thereof. The second surface can be flat, or can besomewhat uneven in a pattern generally corresponding to the array ofcube corner elements on the front surface. For male cube cornerelements, the expression “substrate thickness” means the thickness ofmaterial on which the cube corner elements rest. For female cube cornerelements, the expression “substrate thickness” means the total thicknessof material into which the female cube corner elements form cavities.

The terms “divergence” and “divergence angle” as used herein mean theangle between the direction of the light that enters a retroreflectingelement (e.g., a cube corner) and the direction of the light that leavesthat element. In the context of photometry of retroreflectors, thisangle is commonly called “observation angle”. A non-retroreflecting malecube corner is described by convention as having the divergence of theassociated female cube corner that is the geometric complement of themale and constructed of the same material.

Light retroreflects into a two-dimensional intensity pattern, anddivergence angle measures the distance from the center of this pattern.The expression “range of divergence” as used herein means the range ofdivergence angles for which the retroreflectance pattern is relativelyintense so as to be useful for the intended purpose of theretroreflective article.

The expression “n'th order copy” of some entity as used herein refers tothe resultant of a chain of copies from the first entity, said chaincontaining exactly n−1 intermediate copies. A direct copy is termed a1'st order copy. Copying refers to the copying of the cube cornerarrayed surface geometry of the entities, and it is understood that mostcopying methods produce female arrays from male arrays, and male arraysfrom female arrays. It is further understood that only some portion ofthe last entity needs to be an n'th order copy of only some portion ofthe first entity for the term to apply. If there have been assemblysteps in the chain of copying, many portions of the last entity can ben'th order copies of the same first entity. If one portion of the lastentity is an n'th order copy, while another portion of the last entityis an m'th order copy, of the same first entity, then the last entity isboth an n'th order copy and an m'th order copy of the first entity.Copying can be achieved by electroforming, casting, molding, embossing,and other methods that reproduce the surface geometry at a desiredprecision.

The term “aberration” as used herein means a small change in one or moreof the dihedral angles of a cube corner element, sufficient to cause achange in the divergence of the light retroreflected by the element.

The term “total retroreflectance” as used herein means the amount ofretroreflected light flux within divergence angle 4 degrees, relative tothe incident light flux. The incidence angle is near normal. Theincident light is approximately CIE Illuminant A and the sensing isapproximately CIE V(λ). When determining whether total retroreflectanceis preserved by a process which modifies a retroreflector, compensationis made for any discoloration of the retroreflector. When determiningwhether total retroreflectance is preserved by a process which modifiesa male stage, determination is based on the retroreflectance of femalecopies made from that male.

The term “mean geometrical divergence” as used herein means the averagedivergence angle for all rays retroreflected by a cube corner such aswould be found by raytracing a large number of rays.

FIG. 1A shows a top plan view of a substrate 12 having a first surface13 and a second surface (not shown) opposite said first surface 13, saidfirst surface 13 having formed thereon an array of triangular cubecorner type retroreflector elements 14. The cube corners are males orfemales, depending on how the figure is apprehended.

FIG. 1B shows a front view the substrate illustrated in FIG. 1A when thecubes are males. FIG. 1C shows a front view of the substrate illustratedin FIG. 1A when the cubes are females. In FIG. 1C all the cube cornerdetails are in hidden lines. A convention followed herein makes thesubstrate thickness equal to indicated dimension “t” in FIGS. 1B and 1C.

Cube corner element 14 is formed by three mutually perpendicular faces16 that meet at apex 19. The mutually perpendicular faces 16 intersectone another at dihedral edges 18. The angles at the dihedral edges 18,between the mutually intersecting faces 16, are called dihedral angles.In a geometrically perfect cube corner element, each of the threedihedral angles is exactly 90°.

Generally, for use in the method of the instant invention, the preferredthickness of substrate 12 will depend on the material of which thesubstrate is made. For electroformed nickel, the thickness of thesubstrate 12 will generally be in the range of about 0.1 mm to about 2.0mm, more preferably in the range of about 0.2 mm to about 1 mm, and in apreferred embodiment in a range of about 0.3 mm to about 0.6 mm.Suitable materials from which substrate 12 can be made include metalsand plastics. Substrate 12 can be a ruled master, if that master is nottoo thick. Preferably, substrate 12 is an n'th order copy of a master inwhich cube corner elements are formed such as by ruling, or by any ofthe methods disclosed in the aforementioned U.S. Pat. No. 6,015,214 orSer. No. 10/167,135.

Substrate 12 is then worked in a controlled manner at one or morelocalized regions of its second surface, the working being sufficient tocreate a localized change in stress in the substrate material, whichstress change causes a change in one or more of the dihedral angles ofone or more of the cube corner elements on the first surface oppositethe site of the working. The working of the second surface is intendedto either add, remove, or modify material in the localized regions onthe second surface, or to simply apply local pressure, temperature, orother disturbance. The working is of a degree sufficient to cause achange in the stress of the material comprising the cube corners 14opposite the localized region of the working, resulting in a minutechange of one or more dihedral angles 18, thus aberrating the cubecorner element. This minute change of the dihedral angles will begenerally on the order of a fraction of a degree. For slightly curvedcube faces, the dihedral angles are defined between the planes that bestfit the faces. The degree of face curvature produced by this method canbe optically insignificant.

In a preferred embodiment of the invention, the working on the secondsurface will be of a magnitude sufficiently small that it will notdamage the smoothness of the cube faces and the sharpness of the cubeedges on the first surface. Further, no work need be done on the firstsurface that would adversely affect either the surface smoothness or thesharpness of the edges of the cube corner elements. Therefore, in apreferred embodiment of the invention, the total retroreflectance ofeach cube corner element is substantially preserved.

The controlled working of the second surface that introduces the changein stress can be accomplished by a variety of means, including theapplication of energy, chemicals, machining or pressure to the secondsurface.

Energy can be applied, for example, as either electrical energy orfocused heat, such as by an infra-red laser or a pencil tip torch. Forexample, focused laser energy can melt a very small amount of materialin a localized area on the second surface. The melted material then caneither evaporate, blow away, or resolidify, such that any of thesechanges in the substrate material will either increase or decrease thestress of the material in the localized regions. Laser pulses can beapplied at about 150–700 points per cm² of surface, with each pulseaffecting the dihedral angles of about 1–10 cube corner elements on thefirst surface; other values of the number of laser pulses per unit areacan be used depending upon the desired optical effect.

Chemicals can be particularly useful where the substrate 12 is a plasticmaterial. In such cases, the application of a drop of solvent on thesecond surface of the substrate causes the substrate to pucker slightly,thereby affecting the dihedral angles of at least some of the cubecorner elements on the first surface.

Machining methods can include, for example, micro-drilling. Drillinginvolves the removal of material, with minimal direct disturbance of thesurrounding material. Drilling is preferably used in the method of theinstant invention in which the substrate being worked already has someinternal stress; drilling provides localized relief of stress, and theresulting stress differential creates the minute distortion of thedihedral angles in the cube corner elements on the first surfaceopposite the drilling.

Pressure can produce localized distension involving the movement ofmaterial, such as by contact of the second surface against projectingblunt fingers. Such localized distension preserves material mass whilecreating stress. Such localized distension can be provided by mechanicalmeans, such as by a finger roller that can be rolled against the secondsurface of the substrate. The protruding fingers can be arranged on theroller at about 2–10 fingers/cm², with each fingertip affecting thedihedral angles of about 75 cube corner elements; such finger rollersare commercially available. The working of the second surface isaccomplished by passing the second (flat) surface of the sheeting overthe finger roller to create localized distension in the sheetingsufficient to create changes in the dihedral angles of the cube cornerelements opposite the localized distensions. Copies can be made from theworked article, or the substrate can be the final sheeting article.

The degree of each working of the substrate at each location can beuniform or variable. The variation in the degree of working can bepatterned or it can be semi-random. The locations of the workings alsocan be patterned or semi-random. “Semi-random” refers to a distributionthat is under statistical control, but not controlled in full detail.

One embodiment of the invention is illustrated in FIG. 2, which is aphotomicrograph of a substrate of electroform nickel having a firstsurface and a second surface, with a pattern of female cube cornerelements on the first surface thereof, the substrate having a thicknessof 0.45 mm. Three predetermined localized regions on the second surface,indicated by arrows, spaced 0.5 mm apart, were each subjected to theapplication of a pulsed-focused laser having a wavelength of 1064 nm andpulse duration 4.0 ms supplying an energy of 1.35 J. The cross-sectionalsurface of the sample was polished and then acid etched to reveal grainstructure in the photomicrograph. It can be seen from the revealed grainstructure in FIG. 2 that the laser pulses produced small stresses in thesubstrate material. These small stresses exert an effect through thesubstrate 12 to cause an aberration of the cube corner element at thefirst surface. The effect on the dihedral angles is too small to be seeneven with the photomicrograph, but will have an optically significanteffect on the divergence of the cube corners, and thereby of theretroreflective products that are an n'th order copy of the substrate soworked.

One advantage of the instant invention is that, unlike methods that relyon the grooving to impart dihedral errors, for example the above-citedAppeldom '219 and pending application Ser. No. 10/167,135, the instantmethod can place individual, strongly aberrated cube corners anywhere inthe array surrounded by relatively unaberrated cube corners. Yet anotheradvantage of the disclosed method is that, if the working is alwaysfocused on points on the second surface opposite the apices of cubecorners, the result can be a population of strongly aberrated cubecorners having nearly equal errors on their three dihedral angles, as issometimes desirable. Yet another advantage of the disclosed method isthat it can be used to adjust the divergence characteristics of apreviously made cube corner master, or of an n'th order copy of apreviously made master.

The worked substrate can be used to make retroreflective productsaccording to methods known in the art. For example, after the localizedworking has been completed, several copies can be made of the firstsurface 13 of the substrate 12 and these copies can be assembledtogether either with or without copies of cube corner arrays includingunaberrated cube corner elements, or cube corner arrays having otheraberrations made from other worked substrates. Seamless copies of theassembly can be made such as by electrodeposition of nickel. A copy canbe a tool. Tools made from such arrays of cube corner elements includingone or more aberrated cube corner elements can be used to manufactureretroreflective products such as sheeting. Such manufacturing methodsare known in the art and include, for example, embossing, casting, andcompression molding. A tool of the instant invention can be used in eachof these manufacturing methods and variations thereof. For example, amethod of making a tool is disclosed in U.S. Pat. No. 4,478,769, and amethod of embossing sheeting using such a tool is disclosed in U.S. Pat.No. 4,486,363, both incorporated herein by reference in theirentireties.

The aberration can be applied at any one or more stages in the copyingprocess, to a substrate having either male or female cube corners. Anarray that has been aberrated by the method of localized working can befurther aberrated by re-application of the same method to the samesubstrate as bore the original array. An array that has been aberratedby the method of localized working can be copied one or more times andthat copy can be further aberrated by the same method. Whatever thescheme, in the production of an article there is a last application ofthe method of localized working. Either the article is an n'th ordercopy of the substrate that received the last application of the methodor else the article itself received said last application of the method.

EXAMPLE 1

A nickel electroform 0.38 mm thick having an array of female triangularcube corner elements about 0.1 mm high on its first surface was workedin accordance with the instant invention using laser energy applied atthe second surface directly opposite the apex of a cube corner element.The laser produced infrared radiation having a wavelength of 1064 nm,with a pulse duration of 4.0 ms, supplying 1.4 J of energy. The pulse onthe second surface induced a stress in the electroform, resulting in adistortion of some of the dihedral angles on the first surface. In FIG.3A, the triangular shape of each element is as shown. The small “o”symbol in the interior of each triangle locates the apex of the cubecorner. The number in each corner of each triangular microcube indicatesthe measured deviation in minutes from 90° of the corresponding dihedralangle, wherein 1° equals 60 minutes. The cube corner element oppositethe applied laser is just above the center of the Figure. That cubecorner element nearest the focus of the laser had aberrations in itsdihedral angles ranging from about one-third to about one-half of adegree. It may be seen from FIG. 3A that cube corner elements in thevicinity of the element opposite the pulse of laser energy alsoexperienced aberrations of their dihedral angles, although thoseaberrations were not as great as those experienced by the center cubenearest the focus of the laser pulse. Generally, the dihedral angleaberrations are greater in those cube corner elements that are closer tothe focus of the laser pulse, and are smaller in the cube cornerelements that are farther from the focus of the laser pulse.

It is believed at the present time that a thinner substrate wouldrequire a lower energy laser pulse or other working means to produce anaberration in a cube corner element as shown in FIG. 3A, which wouldresult in smaller aberrations in the neighboring cubes. Similarly, it isbelieved that a thicker substrate would require a higher energy laserpulse or other working means. Therefore, more of that energy would bedissipated to neighboring cubes resulting in greater changes of thedihedral angles of adjacent cube corner elements.

EXAMPLE 2

At another location on the same nickel electroform used in Example 1 thesecond surface was worked in the same manner as in Example 1, exceptthat the focus of the pulse of laser energy was directed to a portion ofthe second surface directly opposite the line between the two cubecorners on the first surface. These two cube corners are shown nearestthe center in FIG. 3B, which figure illustrates aberrations in the samemanner as aberrations were illustrated in FIG. 3A. The two central cubecorners had the greatest measured aberration of their dihedral angles,although the aberration was not as great as for the single cube in thecenter of FIG. 3A.

EXAMPLE 3

At yet another location on the same nickel electroform used in Examples1 and 2, the second surface was worked in the same manner as in Examples1 and 2, except that the focus of the pulse of laser energy was directedto a portion of the second surface directly opposite the point on thefirst surface where six cube corners meet. This point is shown just tothe right of the center of FIG. 3C, which figure illustrates aberrationsin the same manner as aberrations were illustrated in FIG. 3A. Eachdihedral angle emanating from that point showed significant measuredaberration.

Comparative Calculations

Predicted retroreflectance properties of two hypothetical prior artunmetallized acrylic cube corner sheetings are illustrated in FIGS.4A-B, for comparison with predicted retroreflectance properties ofhypothetical unmetallized acrylic cube corner sheeting made inaccordance with the instant invention illustrated in FIG. 4C. FIG. 4Arepresents calculated retroreflectance of a hypothetical acrylicsheeting of the prior art, and being embossed from tooling having apattern of triangular cube corner elements canted 7° edge-more-parallel,0.1 mm deep, and which have no aberration. The mean geometricaldivergence from the acrylic cubes is zero. The three curves of FIG. 4Arepresent calculated R_(A) values for three different rotation angles0°, 45° and 90° of the sheeting.

FIG. 4B represents calculated retroreflectance of a hypothetical acrylicsheeting of the prior art embossed from tooling which differs from thetooling used to make the sheeting of the example of FIG. 4A by havingaberrations introduced during the ruling of the master. Theseaberrations are in the form of dihedral angle errors equal to +9.5minutes on the two shorter dihedral edges and +7.0 minutes on the longerdihedral edge of each triangular cube corner. These unequal dihedralangles were chosen to produce the most nearly symmetrical balance ofgeometrical divergence for the canted triangular cube corner sheeting ofFIG. 4A. When made in acrylic, the mean geometric divergence of cubecorners with these ruled aberrations in their dihedral angles equals41.5 arc minutes. The three curves of FIG. 4B represent R_(A) values forthree different rotation angles 0°, 45° and 90° of the sheeting.

FIG. 4C represents a calculated retroreflectance of a hypotheticalacrylic sheeting embossed from a tooling of the present invention. Thetooling differs from the tooling of the example of FIG. 4A by having apattern of aberrations introduced by laser working on the second surfacein the manner of EXAMPLE 1 and as shown in FIG. 3A, except that (1)instead working the second surface at a single point opposite a singlecube, working is opposite one out of every twelve cube corner elements,(2) aberrations are multiplied by 1.2, to predict the effect of theapplication of a slightly stronger laser pulse, and (3) it is assumedthat the cube corner elements each have a positive error of +2 arcminutes in each dihedral angle before the application of the laserenergy. The mean dihedral angle error of this inventive tooling is then−0.3 arc minutes, and the standard deviation of dihedral angle error is11.7 arc minutes. The mean geometric divergence of acrylic cube cornersheeting made from such tooling is 41.5 arc minutes, identical to theexample of FIG. 4B. Specifically, FIG. 4C shows three curvescorresponding to calculated R_(A) values of the hypothetical cube cornerarray described immediately above measured for three different rotationangles, namely, 0°, 45° and 90°.

FIG. 4D compares the average of the three curves of FIG. 4A, the averageof the three curves of FIG. 4B, and the average of the three curves ofFIG. 4C. It may be seen that while the prior art sheeting of FIG. 4Agives a much higher intensity over a very narrow range of divergenceangle, the retroreflectance values for the cube corner array in whichaberrations have been introduced by the method of the instant inventionas described and illustrated in connection with the embodiment of FIG.4C is predicted to have its intensity extended over a larger usefulrange of divergence angles, up to about 2°. It also may be seen thatwhile the prior art sheeting aberrated in ruling as illustrated in FIG.4B gives intensity similar to the inventive example over the divergenceangle range from about 0.5° to 2.0°, the example of 4B has deficientintensity at the smaller divergence angles rendering it less desirablefor typical applications such as road signs.

In a preferred embodiment of the invention, the working done on thesecond surface will not unduly distort the cube corner elements orunduly impact the smoothness of the optical first surface, such that thetotal retroreflectance of the array is substantially preserved. Whilethe desired percentage of preserved retroreflectance will vary dependingon the total retroreflectance of the original array and the ultimate enduse of the retroreflective article, the total retroreflectance preservedis desirably at least 90%, preferably 94% or better, and most preferably98% or better. When we first used a focused laser on the second surfaceof thin nickel electroforms, some degradation of the smoothness of theoptical first surface seems to have occurred. In one case, the totalretroreflectance from the female electroform was reduced byapproximately 6%. This would imply a similar loss from articles producedfrom the tooling. As of the date of this application, it is believedthat such losses are practically eliminated by working in anon-oxidizing environment, or by sputtering the electroform first with anon-oxidizing coating, such as of gold. At the present time, it isbelieved that the losses observed were due to a surface degradationphenomenon from without, and not due to internal phenomena or to anexcess curvature introduced in the cube faces or edges.

At the present time, when using a focused laser on the second surface ofthin electroforms, the electroforms are left attached to the rigidmandrels on which they were formed. This both excludes air and providesfor rigid support. Total retroreflectance from such electroforms hasbeen found to be reduced by no more than approximately 2% by the lasertreatment. Additionally, it may be useful to machine the second surfaceof the supported thin electroform to a precise uniform substratethickness before the laser treatment, since the effect on the dihedralangles of a given pulse energy is a function of substrate thickness.

Note that when comparing the total retroreflectance of electroformsbefore and after laser treatment, discoloration of the optical surfacemay influence the measurements. We measured plastic copies formed fromthe electroform before and after treatment for their totalretroreflectance and used this to gauge the change to the electroform.

When the inventive technique is practiced using the mechanical removalof material, such as by micro-drilling or micro-machining, it ispreferable that the substrate have internal stress initially. Thematerial removal then achieves localized stress relief of the initiallyuniformly stressed substrate. Subtraction of local stress and additionof local stress are interchangeable for the purpose of the inventivetechnique. For an electroformed substrate, the initial uniform stressingcan be accomplished during its formation, by well-known methods ofplating stress-control. The invention may take on various modificationsand alterations without departing from the spirit and scope thereof. Theinvention will be most useful for structures made by global operationssuch as ruling. For example, although the invention has been illustratedherein using patterns of triangular cube corner elements, cube cornerelements that are square, rectangular, pentagonal, or hexagonal can alsobe used. The invention can also be used with micro-optical systems otherthan cube corners. Accordingly, it is to be understood that the scope ofthis invention is not to be limited to the above-described examples, butis to be controlled by the limitations set forth in the following claimsand any equivalents thereof.

1. A method of producing an array of micro-optical elements, said methodcomprising the steps of: providing a substrate having an array ofmicro-optical elements formed thereon, and controlled working at one ormore localized regions on a surface of the substrate; wherein saidcontrolled working step is performed so as to aberrate one or more ofthe micro-optical elements while preserving total retroreflectance tothereby produce a worked array of micro-optical elements havingdivergence over a desired range and having a total retroreflectance thatis at least 90% of the retroreflectance of the array before saidcontrolled working step.
 2. A method as set forth in claim 1, whereinthe substrate has a first and second opposed surfaces and wherein thefirst surface has the array of the micro-optical elements formedthereon, and wherein the second surface is the surface of the substrateon which the controlled working is performed.
 3. A method as set forthin claim 1, wherein the substrate is a previously tooled master.
 4. Amethod as set forth in claim 1, wherein the substrate is an nth ordercopy of a tooled master.
 5. A method as set forth in claim 1, whereinthe micro-elements are cube corner elements and wherein said controlledworking step is performed so as to change one or more dihedral angles ofone or more of the cube corner elements.
 6. A method as set forth inclaim 5, wherein the cube corner elements are triangular cube cornerelements.
 7. A method as set forth in claim 5, wherein the magnitude ofthe working is sufficiently small such that smoothness of faces of thecube corner elements and/or sharpness of the dihedral edges of the cubecorner elements is not substantially damaged.
 8. A method as set forthin claim 5, wherein said controlled working step is performed so as tohave a population of strongly aberrated cube corner elements each havingnearly equal errors on all of its dihedral angles.
 9. An articlecomprising a substrate for making an array of retroreflective elements;the substrate having an array of micro-optical elements formed thereon;the array of micro-optical elements including at least one aberratedmicro-optical element and a plurality of non-aberrated micro-opticalelements; the substrate having a surface having localized regions whichhave been controlled worked to form the aberrated micro-opticalelements; the array of micro-optical elements having divergence over adesired range and having a total retroreflectance that is at least 90%of the retroreflectance of the array if the localized regions had notbeen controlled worked.
 10. An article as set forth in claim 9, whereinthe substrate has a first surface on which the micro-optical elementshave been formed and a second surface opposed to the first surface, andwherein the second surface is the surface of the substrate having thelocalized regions which have been controlled worked to form theaberrated micro-optical elements.
 11. An article as set forth in claim9, wherein the substrate is a previously tooled master.
 12. An articleas set forth in claim 9, wherein the substrate is an nth order copy of atooled master.
 13. An article as set forth in claim 9, wherein the arrayis undisturbed except for the aberrational changes within the individualaberrated micro-optical elements of the array.
 14. An article as setforth in claim 9, wherein aberrated micro-optical elements aresurrounded by non-aberrated micro-optical elements.
 15. An article asset forth in claim 9, wherein the micro-elements are cube cornerelements and wherein the aberrated cube corner elements have one or moredihedral angles altered by the controlled working at the localizedregions.
 16. An article as set forth in claim 15, wherein the cubecorner elements are triangular cube corner elements.
 17. An article asset forth in claim 15, wherein the smoothness of faces of the cubecorner elements and/or the sharpness of the dihedral edges of theaberrated cube corner elements is not substantially damaged.
 18. Anarticle as set forth in claim 15, wherein the array has a population ofstrongly aberrated cube corner elements each having nearly equal errorson all of its dihedral angles.
 19. A method of making a retroreflectiveproduct, said method comprising the steps of: copying the article setforth in claim 9; assembling the copy or copies of the article togetherinto a tool; and using the tool to manufacture the retroreflectiveproduct.
 20. A method of making a retroreflective product, said methodcomprising the steps of: providing a substrate having an array ofmicro-optical elements formed thereon, controlled working at one or morelocalized regions on a surface of the substrate, said controlled workingstep being performed so as to aberrate one or more of the micro-opticalelements while preserving total retroreflectance to thereby produce aworked array of micro-optical elements having divergence over a desiredrange and having a total retroreflectance that is at least 90% of theretroreflectance of the array before said controlled working step;assembling the substrate, and/or copies of the substrate into a tool;and using the tool to manufacture the retroreflective product.